The polis (πόλις) literally means "city" in Greek. The plural is "poleis" (πόλεις). It's almost synonymous with "city-state": a sovereign state, also described as a type of small independent country, that usually consists of a single city and its dependent territories. Athens was one, for example, as was Sparta.

In the ancient world, the polis was a nucleus, the central urban area that could also have controlled the surrounding countryside, which were usually part of the polis. These outskirts were called usually called "Khôra" (χώρα). There were around 1500 archaic and classical Hellenic poleis. Many of them are listed here. The region formed by a cluster of poleis, bound geographically and ethnically, was an ethnos (ἔθνος, nation). Its plural is "ethne."

The Ancient Hellenic city-state developed during the Archaic period as the ancestor of city, state, and citizenship and persisted (though with decreasing influence) well into Roman times. The term polis, which in archaic Greece meant "city", changed with the development of the governance center in the city to signify "state" (which included its surrounding villages). Finally, with the emergence of a notion of citizenship among landowners, it came to describe the entire body of citizens. The ancient Hellenes did not always refer to Athens, Sparta, Thebes, and other poleis as such; they often spoke instead of the Athenians, Lacedaemonians, Thebans and so on. The body of citizens came to be the most important meaning of the term polis in ancient Hellas.

Plato analyzes the polis in The Republic, whose Greek title, Πολιτεία (Politeia), itself derives from the word polis. The best form of government of the polis for Plato is the one that leads to the common good. According to Plato, there are five main economic classes of any polis: producers, merchants, sailors/shipowners, retail traders, and wage earners. Along with the two principles and five economic classes, there are four virtues. The four virtues of a "just city" include, wisdom, courage, moderation, and justice. With all of these principles, classes, and virtues, it was believed that a "just city" (polis) would exist.

Derivatives of polis are common in many modern European languages. This is indicative of the influence of the polis-centred Hellenic world view. Derivative words in English include policy, polity, police, and politics. In Greek, words deriving from polis include politēs and politismos, whose exact equivalents in Latin, Romance, and other European languages, respectively civis ("citizen"), civilisatio ("civilization"), etc., are similarly derived.